DIDYMOaRAPTUS. 43 



aperture, which is clearly visible. Thus the second stipe apparently originates 

 nearly at the same level as the first. 



The thecal number fifteen to thirteen in 10 nnn. ; they are long, narroAV tubes, 

 and when mature their length is about four times as great as their width at the 

 broadest part ; they are free one third to a quarter their length in the distal 

 part of the sti})es, but in the proximal portion they may be free for nearly half 

 their length. The distal tliecfe have a length of about 3 mm., and are inclined 

 at an average angle of 45°, but the thecal Avails are curved in such a manner 

 that the angle of inclination near the aperture may be rather higher than this. 

 In the initial region the angle is much lower, and rarely exceeds 30°. 



Remarks. — D. bifidas exhibits great variation in the angle of divergence of the 

 stipes ; indeed, there may almost be said to be two distinct types included in it, 

 one in which the stipes diverge at an angle of about ()0° (PL IV, fig. 1 /), 

 and another in which the base is rounded, the initial angle of divergence 

 approaches a right angle, and the stipes are ultimately approximately jjarallel, 

 or include between them a very small angle (PI. IV, fig. 1 c). These 

 extremes are, however, connected by a long series of intermediate forms, and 

 agree in all the relations of the earliest tliecje to the sicula, and in the 

 characters of the theca? in the distal parts of the stipe ; it is impossible, therefore, 

 to draw a line between them, and they are included in one variable species. 

 The commonest form is that in which the base is rounded, and the stipes 

 include between them angles of from 15° to 20°. 



The species is characterised by the following features : 



(1) the form of the sti}>es, with straight dorsal walls and curved 



celluliferous margins, 



(2) the gradual and persistent widening from their origin, 



(3) the characters of the thecae and their number in a given unit of length. 

 Affinifies. — D. hifidus is closely related to the typical form of D. Murchisoni, 



and to var. geminus. From the former, with which it agrees in the number of 

 thecse in a given unit of length, it may he distinguished by the divergent stipes 

 and their persistent increase in width; from var. (/eminM><, which it resembles 

 in the divergence of the stipes, it may be separated by their persistent widening 

 and the numljer of thecal in a given unit of length. 



D. hifidiis is more a1)undant in Britain than any other member of the dependent 

 series ; it occurs in great numbers at one i)articular horizon, namely, in the Upper 

 Arenig beds below the zone of D. Murchisoin. 



Horizon aiul Localities. — Upper Arenig (Llanvirn of Hicks). Middle Arenig 

 (S. Shropshire). Ireland — Go. Mcafh: Bellewstown; Co. Kildarc : Grange Hill. 

 S. Scoiland — Ayrshire : Bennane Head, Ballantrae. England, — Lake District : 

 Harf ; Doddick Fell; Saddleback; Outerside ; Burstock Scar; SkiddaAV ; Thorn- 

 slii[) Beck, Shap ; Ellergill ; Aik BecK ; E. Dodd AVood ; Shropsliire : Shelve; 



